Methods of electrostatic control in semiconductor devices

ABSTRACT

A semiconductor light-emitting device having one or more depletion regions that are controlled by one or more control electrodes to vary the spatial distribution of the carriers in an active layer. The voltages on the control electrodes can be controlled to modulate the current density in the active layer and the output light intensity. The polarization of a surface emitting diode laser based on this device can be controlled or modulated.

This application is a divisional application of U.S. patent applicationNo. 08/990,145 filed on Dec. 12, 1997, now U.S. Pat. No. 6,040,590, andclaims the benefit of U.S. Provisional Application No. 60/032,660 filedon Dec. 12, 1996, which is incorporated herein by reference.

ORIGIN OF THE INVENTION

The U.S. Government has certain rights to this invention pursuant toGrant No. N00014-96-1-1295 awarded by the Advanced Research ProjectsAgency.

FIELD OF THE INVENTION

The present invention relates to semiconductor optoelectronic devices,and more specifically, to light-emitting semiconductor devices.

BACKGROUND OF THE INVENTION

Many semiconductor light-emitting devices produce light throughradiative recombination of electrons and holes at a p-n junction. Bothbulk materials and quantum well structures may be used to form p-njunctions. Quantum well p-n heterojunctions can produce highlight-emitting the efficiency of this type of Examples of suchlight-emitting devices include light-emitting devices. In operation, adriving electrical current is applied to the p-n junction to injectcarriers (i.e., electrons and holes) with energy above their respectiveequilibrium level into the p-n junction. A large portion of theelectrons and holes recombine to release the excessive energy as light.The remaining electrons and holes recombine through non-radiativeprocesses to produce heat. From another point of view, a majority of theelectrons that are excited from the valence band to the conduction bandby absorbing the energy from the driving current radiate photons bydecaying back to the valence band.

Light-emitting diodes (“LEDs”) and diode lasers. LEDs operate based onthe spontaneous emission of photons and diode lasers operate based onthe stimulated emission of photons and population inversion. Thestructure of a diode laser is usually more complex than that of a LEDsince an optical cavity is required in a diode laser to providenecessary optical feedback for laser oscillations.

It is desirable to generate as much light as possible for a given amountof driving current in both LEDs and diode lasers. This aspect of a LEDor diode laser can be characterized by electrical-to-light conversionefficiency which is defined as the ratio of the output light power tothe injected electrical power. In practical devices, increasing theelectrical-to-light conversion efficiency can also reduce the heatcaused by the remaining electrical energy from the injected drivingcurrent that is not converted into light. Low thermal dissipation isparticularly desirable in manufacturing compactly integrated photoniccircuits.

The electrical-to-light conversion efficiency of a LED or diode laserhas an upper limit defined by the internal quantum efficiency of a p-njunction, which is the rate of emission of photons divided by the rateof supply of electrons (or holes). Choice of semiconductor materials,dopants and respective doping concentrations may be used to increase thequantum efficiency. Use of quantum well structures rather than bulkmaterials to form a p-n heterojunction, for example, is one approach toimprove the quantum efficiency.

The device structure of a LED or diode laser may also affect theelectrical-to-light efficiency. For a given p-n junction, theelectrical-to-light efficiency is mainly determined by the devicestructure since the quantum efficiency is essentially fixed. Variousstructures for LEDs and diode lasers have been developed to improve theelectrical-to-light efficiency. One effort in this area is to confinethe driving current to a small spatial region at or near the active p-njunction in order to increase the current density in the p-n junction.This results in an increase in the rate of supply of electrons (orholes) to the p-n junction, and thereby increases the rate of photonemission.

In diode lasers, the electrical-to-light efficiency can be representedby a laser threshold current at which a population inversion is createdbetween the conduction and valence bands. At this threshold current, theoptical gain caused by the driving current is equal to the total opticalloss. A low threshold current indicates a high electrical-to-lightconversion efficiency. For a given active p-n junction with a fixedthickness and material compositions, the laser threshold current densityis also given. Thus, confining current spatial distribution to a smallerregion near or at the p-n junction increases the corresponding currentdensity and effectively reduces laser threshold current.

SUMMARY OF THE INVENTION

The present disclosure provides electrically adjustable carrierconfinement in semiconductor light-emitting devices (e.g., LEDs or diodelasers) based on electrostatic control of carriers. Such semiconductorlight-emitting devices have at least one depletion-producing element toproduce a depletion region to confine spatial distribution of thecarriers within an active semiconductor medium. This electrostaticcontrol can be used to change the current density and to implement acontrol mechanism.

One embodiment of the invention uses a reverse biased Schottky contactto produce the depletion region.

Another embodiment of the invention uses a MOS capacitor to produce thedepletion region.

Yet another embodiment uses a reverse biased p-n junction to produce thedepletion region.

The electrostatic control can be configured to be independent of theoptical confinement in an index-guided diode laser. Under such aconfiguration the laser operation may be controlled by separatelyconfiguring the carrier confinement and the optical confinement.

The depletion region may be adjusted to modulate certain properties ofthe output light such as output optical power in a semiconductorlight-emitting device, laser polarization and threshold current of adiode laser.

These and other aspects and advantages of the invention will become moreapparent in light of the following detailed description, including theaccompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an edge-emitting device with Schottkycontacts.

FIGS. 2A, 2B, 2C, 2D and 2E are cross sectional views of variousembodiments of a vertical cavity surface emitting laser with anelectrostatic control.

FIG. 3 is a diagram showing one embodiment of a surface-emitting LED.

FIG. 4 is a diagram showing an edge-emitting device with a MOS capacitorstructure for electrostatic control.

FIG. 5 is a diagram showing a vertical cavity surface emitting laserwith a MOS capacitor structure for electrostatic control.

FIG. 6 is a diagram showing an edge-emitting device with a reversebiased structure for electrostatic control.

FIG. 7 is a diagram showing a vertical cavity surface emitting laserwith a reverse biased structure for electrostatic control.

FIG. 8 is diagram showing a circular configuration of symmetricallyarranged multiple control electrodes in a vertical cavity surfaceemitting laser for polarization control.

FIG. 9A shows one implementation of the edge-emitting device structureshown in FIG. 1.

FIGS. 9B, 9C, 9D and 9E are charts showing measured data obtained fromlasers using the implementation of FIG. 9A.

FIGS. 10A and 10B are diagrams showing alternative embodiments in whichan active layer is located adjacent to control electrodes.

FIG. 11 is a diagram showing an example of a different mesa geometry.

FIG. 12 is a diagram showing a configuration for edge-emitting devicesand vertical cavity surface emitting lasers without a mesa structure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention uses an adjustable electrostatic field within aLED or a diode laser to confine spatial distribution of the carriers andto implement a control mechanism. The carrier confinement can beconfigured to be independent of the optical confinement in anindex-guided diode laser so that the laser operation may be controlledby separately configuring the carrier confinement and the opticalconfinement. In addition, the electrostatic field may be adjusted tomodulate certain properties of the output light and to vary the laserthreshold current of a diode laser.

FIG. 1 is a cross-sectional diagram showing one embodiment of anedge-emitting LED or diode laser according to the invention. An activesemiconductor layer 110 is sandwiched between first and second barrierlayers 112 a and 112 b to form an active light-emitting medium 102. Atleast one of the first and second barrier layers 112 a and 112 b is madeof a semiconductor material with a bandgap larger than that of thecenter active layer 110. Preferably, both barrier layers 112 a and 112 bhave bandgaps larger than that of the center layer 110 to form a doubleheterojunction. The thickness of the center layer 110 may be reduced toa small value and the barrier layers 112 a and 112 b may be configuredto form a single-crystal lattice with the center layer 110 so that theactive medium 102 is a quantum well double heterojunction. The activemedium 102 may also have two or more quantum well double heterojunctionstructures stacked together to form a multiple quantum structure.

Two cladding layers 114 and 118 having bandgaps larger than the centerlayer 110 are respectively formed on the sides of the active medium 102to provide optical confinement in a direction perpendicular to thesemiconductor layers since the indices of refraction of the claddinglayers 114 and 118 are smaller than the center layer 110. The activemedium 102 and the cladding layers 114 and 118 are formed on a substrate116 so that the device 100 is monolithically integrated.

A ridge-like or mesa-like strip 118 a may be formed on top of the secondcladding layer 118 to effect an optical confinement due to the indexvariation in the direction parallel to the semiconductor layers. Ohmiccontacts 120 and 122 are respectively formed on the semiconductor mesa118 a and the substrate 116 to inject carriers into the active layer 110as electrical excitation and to provide a forward bias in the p-njunction in the active medium 102. This can be done by, for example,connecting the ohmic contacts 120 and 122, to an electrical currentsource. The ohmic contact 120 can be a strip due to the mesa-like strip118 a and limits the current distribution along the semiconductorlayers. Therefore, the mesa-like strip 118 a effects both opticalconfinement and carrier confinement.

Preferably, the semiconductor layers on one side of the active layer 110may be doped to exhibit a conducting type (e.g., p-type) different fromthe semiconductor layers located on the other side (e.g., n-type).Therefore, if the first barrier layer 112 a, the first cladding layer114, and the substrate 116 are n-doped, the second barrier 112 b, thesecond cladding layer 118 and the mesa 118 a are then p-doped.Accordingly, the electrical potential of the ohmic contact 120 should behigher than that of the ohmic contact 122 so that a current is injectedto flow from the ohmic contact 120 to the ohmic contact 122.

The embodiment 100 further implements two metallic control electrodes130 a and 130 b that are directly formed over the second cladding layer118 and the sides of the mesa strip 118 a. Alternatively, the controlelectrodes 130 a and 130 b may also be located either on the sides ofthe mesa strip 118 a only or on the surfaces of the second claddinglayer 118 only The control electrodes 130 a and 130 b may besymmetrically located with respect to the ohmic contact 120 and areinsulated from the ohmic contact 120. The interface of the controlelectrodes 130 a or 130 b with the second cladding layer 118 and themesa strip 118 a effects a Schottky contact. Thus, a depletion regioncan be created within the second cladding layer 118 and the mesa strip118 a by applying a control voltage to, apply a reverse bias theSchottky contact. The reverse biased Schottky contacts arenon-conducting and each produce an electrostatic field gradient in therespective depletion region to repulse the carriers. Since such aSchottky contact is formed on both sides of the mesa strip 118 a, thecarriers are confined within a reduced volume in the central regionbetween the depletion regions. For a given driving current, the currentdensity in the active medium 102 is effectively increased.

Therefore, the control electrodes 130 a and 130 b can be used to controlthe spatial distribution of the carriers and thereby change the currentdensity within the active medium 102 without changing the drivingcurrent. When the device 100 is configured to operate as a LED, thecontrol voltage applied to the control electrodes 130 a and 130 b may bevaried to modulate the output light intensity.

The device 100 can be configured to operate as an edge-emitting laser byforming an optical cavity with the cavity optic axis along the directionof the mesa strip 118 a. The control electrodes 130 a and 130 b can beused to change the laser threshold current by varying the spatialconfinement of the carriers. For example, as the bias across theSchottky contact increases within a certain range, the depletion regionproduced by each control electrode increases, resulting in a reducedspatial profile of the driving current. This causes the laser thresholdcurrent to decrease since the current density in the active medium 102is increased. Hence, the device 100 may be used to achieve a reducedlaser threshold. Also, the control electrodes 130 a and 130 b may beused to provide a laser switch to turn on or off the laser oscillationby varying the current density above and below the laser threshold.

Furthermore, the control electrodes 130 a and 130 b may be used tomodulate the output power of a laser based on the device 100. Thedriving current provided by the ohmic contacts 120 and 122 is maintainedat a level so that the current density in the active medium 102 is abovethe laser threshold for a predetermined control voltage on the controlis electrodes 130 a and 130 b. Under this condition, the laser isactivated and produces a fixed laser power. When the control voltage ischanged within a range to alter the current density within the activemedium 102 above the laser threshold, the output of the laser is alsochanged. Therefore, a modulation on the control voltage to the controlelectrodes 130 a and 130 b produces a modulation in the output laserpower. Such a current modulation has simpler circuitry than many currentmodulation circuits that control the driving current since only thevoltage is modulated. In addition, the response speed can be very fastsince the RC time constant of the control electrodes can be maderelatively small.

The Schottky contacts formed by the control electrodes 130 a and 130 bwith the cladding layer 118 and the mesa strip 118 a are reverse biasedin order to achieve the above carrier confinement and operation control.This desired condition requires that the control voltage on each controlelectrode (130 a and 130 b) be at a value so that the electricalpotential of each control electrode is higher than that of the ohmiccontact 120 if the second cladding layer 118 and the second barrierlayer 112 b are p-doped and the electrical potential of each controlelectrode is lower than that of the ohmic contact 120 if the secondcladding layer 118 and the second barrier layer 112 b are n-doped. Forexample, when the first barrier 112 a, the second cladding layer 114 andthe substrate 116 are n-doped and the semiconductor layers on the otherside of the active layer 110 are p-doped, the ohmic contact 120 can beat a positive voltage and the ohmic contact 122 can be grounded toprovide a forward bias to the active layer 110. In this configuration,the potential on either of the control electrodes 130 a and 130 b shouldbe higher than the positive voltage on the ohmic contact 120.

The device 100 of FIG. 1 effectively forms a field effect transistor(“FET”) where the ohmic contracts 120 and 122 function as the source ordrain terminals and the control electrodes 130 a and 130 b function asthe gate for the FET. Since the gate is reverse biased, the gate doesnot change the magnitude of the driving current but rather controls thedimension of the conduction channel for the driving current.

The above electrostatic control can also be implemented invertical-cavity surface emitting lasers (“VCSLs”) and surface emittingLEDs. FIGS. 2A-2E and FIG. 3 show several embodiments in accordance withthis aspect of the invention.

FIG. 2A shows a VCSEL 200 having a mesa formation 118 b on top of thesecond cladding layer 118 and a control electrode 210. Similar to thecontrol electrodes 130 a and 130 b in the device 100 of FIG. 1, thecontrol electrode 210 forms a reverse-biased Schottky contact withcladding layer 118 and the mesa 118 b to provide electrostatic controlof the carrier distribution. An ohmic contact 210 on top of the mesa 118b may be transparent or may have an aperture (e.g., a central aperture)to allow for transmission of light. An optical cavity is formed by twooptical reflectors 230 and 240 to have a cavity optic axis perpendicularto the semiconductor layers. The reflector 230 is formed on top of theohmic contact 220 and can be made of any suitable optical reflectiveelement. A reflector having multiple dielectric layers (e.g., adistributed Bragg reflector) may be used as the reflector 230. The otherreflector 240 may be a stack of alternating quarter-wavelengthsemiconductor layers (also a distributed Bragg reflector) of differentindices at a selected laser wavelength and can be formed between thefirst cladding layer 114 and the substrate 116. Alternatively, theindices of the layers in the reflector 240 may be chosen to be smallerthan that of the active layer 110 so that the reflector 240 may alsoprovide optical confinement and the first cladding layer 114 may beeliminated. As previously, the active medium 102 may be a quantum wellstructure.

FIG. 2B shows a VCSEL 202 in which a reflector 230 a is formed betweenthe ohmic contact 220 and the mesa 118 b. The reflector 230 a may be adistributed Bragg reflector made of a stack of alternatingquarter-wavelength semiconductor layers of different indices.

FIG. 2C shows a VCSEL 204 in which a reflector 230 b is used toconstruct a mesa formation to accommodate the control electrode 210. Thereflector 230 b may be a distributed Bragg reflector having alternatingquarter-wavelength semiconductor layers of different indices. The secondcladding layer 118 may be eliminated if the indices of thequarter-wavelength semiconductor layers are higher than that of theactive layer 110.

FIGS. 2D and 2E show other configurations of VCSELs with a controlelectrode 210. In FIG. 2E, the reflector 240 b may be made of anysuitable optical reflective element. However formed, a VCSEL has anoptical cavity enclosing the active layer 110 with the cavity optic axisperpendicular to the semiconductor layers.

FIG. 3 shows a surface-emitting LED 300 based on the above electrostaticcontrol. A lightly p-doped semiconductor layer 310 and a n-dopedsemiconductor layer 312 forms a light-emitting p-n junction over ap-substrate 314. The n-doped layer 312 has a mesa 316 to accommodate thecontrol electrode 210. The control voltage on the control electrode 210can be varied to change the current density and thereby the output lightintensity. An optical reflector may be optionally formed on the p-dopedside of the p-n junction and redirect light emitted therein to n-dopedlayer 312 to increase the output intensity. For example, a Braggreflector may be formed between the layers 310 and 314.

The electrostatic control produced by a reverse biased Schottky contactmay also be produced by a metal-oxide-semiconductor (“MOS”) capacitorformed in a light-emitting device (e.g., a LED or diode laser). FIGS. 4and 5 respectively show an edge-emitting device 400 and a VCSEL 500 thatimplement an electrostatic control with a MOS capacitor.

Referring to FIG. 4, a semiconductor cladding layer 410 is formed toprovide an electrostatic control in the edge-emitting device 400. Thecladding layer 410 is a mesa-like strip and is made of a semiconductormaterials with an index of refraction smaller than that of the activelayer 110 of the active medium 102. Two ohmic contacts 120 and 122 areformed to inject a driving current to excite the active medium 102.

An oxidation layer 412 is formed on each side of the mesa strip 410 by,for example, using a known oxidation process or directly depositing anoxide layer. A control electrode 420 is then formed on each side of themesa strip 410 over the respective oxidation layer 412 and iselectrically insulated from the ohmic contact 120. Each controlelectrode 420 is applied with a control potential that causes a reversebias across the control electrodes 420 and the second cladding layer410. Hence, when the second cladding layer 410 is p-doped, the controlpotential is higher than the electrical potential of the ohmic contact120; the control potential is lower than the electrical potential of theohmic contact 120 when the second cladding layer 410 is n-doped. Thisconfiguration creates a non-conducting depletion region within the mesastrip cladding layer 410 near the oxidation layer 412. The depletionregions near both sides of the mesa strip cladding layer 410 confine thecarriers in the central region. The size of the depletion regions can bevaried by changing the control potential. Similar to the Schottkyscheme, this provides a control over the current density and can be usedto modulate the output light power and to change the threshold currentin a laser.

FIG. 5 shows a VCSEL 500 using the MOS capacitor control. Other VCSELconfigurations, including those shown in FIGS. 2B-2E, are also possible.

The devices shown in FIGS. 4 and 5 also effect a metal-oxide fieldeffect transistor (“MOSFET”) where the ohmic contracts 120 and 122function as the source or drain terminals and the control electrodes 420function as the gate for the MOSFET. Since the gate is reverse biased,the gate does not change the magnitude of the driving current but rathercontrols the dimension of the conduction channel for the drivingcurrent.

It is further contemplated that a reverse biased p-n junction may alsobe used to provide an electrostatic control. FIG. 6 shows anedge-emitting semiconductor device 600 according to this aspect of theinvention. An active layer 610 and two cladding layers 612 and 616 areformed on a substrate 614. The cladding layer 616 may be doped to have adifferent conducting type (i.e., n- or p-doped) than that of thecladding layer 612 and the substrate 614 (i.e., p- or n-doped). FIG. 6shows an example that the cladding layer 616 is p-doped for simplicityof description. A heavily p-doped layer 618 is formed over the claddinglayer 616 for forming an ohmic contact 620.

The cladding layer 616 is processed to form a ridge structure sandwichedbetween two n-doped regions 618 a and 618 b. Two p-n junctions areformed on both sides of the ridge structure of the cladding layer 616with the n-doped regions 618 a and 618 b, respectively. Two controlelectrodes 630 a and 630 b respectively formed on the n-doped regions618 a and 618 b are electrically insulated from the ohmic contact 620and provide a proper reversed bias to the respective p-n junction.Insulating regions 616 a and 616 b (e.g., oxidation layers) may beformed to prevent electrical breakdown.

The electrical potential applied to either 630 a or 630 b is higher thanthat of the ohmic contact 620 to implement the reversed bias in each p-njunction. This produces a non-conducting depletion region within thecladding layer 616 near each p-n junction and confines the carriers toflow through the central region between the depletion regions. Thedimension of the depletion regions can be varied by the controlpotential to change the spatial distribution of the carriers and thecurrent density. Similar to Schottky contact scheme and MOS capacitorscheme, a control of laser threshold current and a modulation of theoutput light power can be achieved.

FIG. 7 shows a VCSEL 700 using the reverse biased p-n junctions toimplement the electrostatic control. Other VCSEL configurations are alsopossible.

In the above-described VCSELs with electrostatic control based on theSchottky contact scheme, MOS capacitor scheme or the p-n junctionscheme, the depletion region or regions are preferably symmetric withrespect to the optic axis of the optical cavity formed by the tworeflectors. In particular, two or more symmetrically located depletionregions may be used to modulate or control the output polarization ofthe laser oscillation.

FIG. 8 shows one exemplary arrangement of multiple control electrodesfor polarization control in accordance with this aspect of theinvention. Four control electrodes 810 a, 810 b, 820 a, and 820 b aresymmetrically disposed to form a circular arrangement. Any two adjacentcontrol electrodes are separated and electrically insulated from eachother. Each control electrode corresponds to a depletion region producedby a Schottky contact, a MOS capacitor, or a p-n junction formedthereunder. Any pattern arrangement other than the illustrated circularpattern may be used. The geometry of the control electrodes and theunderlying Schottky contacts, MOS capacitors or p-n junctions may alsobe any suitable shape and dimension.

In operation, the electrical potentials applied to the multiple controlelectrodes may be adjusted to achieve a desired carrier distributionpattern. When the cross sectional shape of the carrier distribution iselongated in a selected direction, the optical gain for polarizationsalong that selected direction will be increased and the gain forpolarizations orthogonal to that selected direction will be decreased.This produces an output laser beam with modes polarized along thatselected direction.

In the example shown in FIG. 8, the opposing control electrodes 810 aand 810 b may be applied with voltages less than the voltages applied tothe other two opposing control electrodes 820 a and 820 b to produce anelliptical carrier distribution 830. This carrier distribution producesan output laser beam polarized along the major axis of the ellipse asshown by the arrow “E”.

Therefore, by controlling the spatial carrier distribution, the gain foran electric field polarized along a given direction can be controlled,without changing the total number of carriers. This technique can beused to stabilize the polarization of VCSELs which would be otherwiseunstable and oscillate randomly. In addition, by applying proper varyingcontrol voltages to the control electrodes, the carrier distribution canbe modulated to produce a modulation on the output polarization.Further, an amplitude modulation can be accomplished by passing thepolarization-modulated output light from this type of VCSELs through apolarizer.

High-speed modulations can be achieved using this scheme since the RCtime constant of the control electrodes and photon lifetime in thecavity can be made very short. Frequency chirp in such modulation can besignificantly reduced since the total carrier density is constant.

The electrostatic control described above can also be used to controlthe transverse mode profile of the output beam in both edge-emittingconfiguration and the VCSEL configuration by changing the carrierdistribution. For example, the output beam profile can be controlled forcoupling the beam into an optical fiber.

FIG. 9A shows a cross sectional view of one implementation of theelectrostatic control based on the Schottky contact scheme. The device900 is an edge-emitting diode laser. The structure is grown on a n-dopedsilicon GaAs substrate 916 by using molecular beam epitaxy. An activelayer 910 has a single 10-nm GaAs quantum well within a graded indexseparate confinement heterostructure (“GRINSCH”) with barriers gradedbetween Al_(0.3)Ga_(0.3)As and Al_(0.2)Ga_(0.8)As. The active layer 910is sandwiched between two adjacent cladding layers 914 and 915 which arerespectively n-doped and p-doped AlGaAs layers of about 2 μm inthickness. Doping in the n-type cladding layer 914 is about 3×10¹⁸ cm⁻³(Si). The p-AlGaAs layer 915 is lightly doped with Be at about 3×10¹⁵cm⁻³ to provide a low doped region for the Schottky contact. A gradedp-Al_(0.5)Ga_(0.5)As cladding layer 918 of about 0.33 μm is formed ontop of the cladding layer 915 and the doping level of Be is linearlyincreased to about 3×10¹⁸ cm⁻³. A graded p-GaAs contact layer 919 ofabout 0.33 μm is further formed over the cladding layer 918 with Bedoping graded from about 3×10¹⁸ cm⁻³ to about 1×10¹⁹ cm⁻³ in order toprovide good ohmic contact. A first ohmic p-contact 920 made of Cr/Au isformed on the contact layer 919 and a second ohmic n-contact 922 made ofAuGe/Au is formed on the substrate 916. Electrodes 930 (Cr/Au) form theSchottky contacts with the cladding layers 918 and 915.

The layers 918 and 919 are configured to form a ridge waveguide by usinga chemically assisted ion beam etch (“CAIBE”) with Cl₂/Ar⁺. Thisanisotropic etching is performed at approximately 35 degrees from thesurface normal from either side of a photoresist strip mask until thelightly p-doped layer 915 was exposed. The undercutting of thephotoresist mask allows for the self-aligned deposition of Cr/Auelectrodes 920 and 930. Alternatively, the ridge waveguide may also beformed by performing an anisotropic wet etch with a suitable etchantsolution (e.g., a 1:8:40 solution of H₂SO₄:H₂O₂:H₂O) to provide thenecessary undercut. Argon ion milling is used with a photoresist mask toelectrically isolate the device. Finally, the wafer is lapped down toapproximately 6 mils and an AuGe/Ni/Au layer was applied on thesubstrate 916 to form the ohmic contact 922.

FIG. 9B is a chart showing the measured output power as a function ofthe driving current for a 13-μm wide laser. Only one control electrodewas applied with a control voltage. The laser was pumped with currentpulses of 250 ns at 30 kHz. A 4%˜5% reduction in laser threshold currentwas observed when a control voltage of about 4 V was applied on one ofthe control electrodes. The depletion range change due to this 4-Vvoltage was estimated to be about 0.5 μm to about 0.6 μm. When thecontrol voltage is applied to both of the control electrodes, the effectcan be expected to approximately double. The performance of theelectrostatic control can be further improved by improving the laserstructure.

The leakage current through the reverse biased Schottky contact was alsomeasured with a laser of 25 μm wide and 500 μm long. This is shown inFIG. 9C. The measured current leakage corresponds to a leakage currentdensity of less than 32 mA/cm². Broad area laser threshold density forthis material is about 260 A/cm². The data indicates that the leakagecurrent through the Schottky contact produced by the control electrodeis negligibly small compared to the pump current and could not accountfor the increase in the output power.

FIG. 9D shows the measured laser output spectrum for a control voltagescanning from 0 V to about 20 V. A laser of about 60 μm wide and 600 μmlong was used in the measurements. The pump current was controlled at aconstant slightly above the threshold. The laser operated in a multimodeconfiguration. A significant increase in the output power was measuredas the control voltage increased without measurable wavelength shift.More specifically, the peak output power at the control voltage of 20 Vis more than double the peak power when the control voltage is at zero.This indicates that the effect of the electrostatic control with thedepletion region on the round trip cavity phase shift is small enoughnot to significantly affect the lasing spectrum under the resolution ofthe measurement system. The data further indicates that the depletionregion mainly affects the carrier distribution within the laser cavity.

FIG. 9E further shows the variation in the integrated power with thecontrol voltage on the Schottky contacts. The measurements were takenfrom the same laser under the same operating condition as themeasurements shown in FIG. 9D. The optical power increases by a factorof approximately 1.5 for an increase from 0 v to 25 V in the controlvoltage applied only to one control electrode.

Although the present invention has been described in detail withreference to a few embodiments, one ordinarily skilled in the art towhich this invention pertains will appreciate that various modificationsand enhancements may be made without departing from the scope and spiritof the invention. For example, the active layer may be formed closer tocontrol electrodes in order to more effectively confine the carrierdistribution within the active layer. FIGS. 10A and 10B illustrate thisconfiguration in the Schottky contact scheme (FIG. 10A) and the reversebiased p-n junction scheme (FIG. 10B).

The geometry of the mesa cladding layers may be made different from theslanted shapes as described. For Example FIG. 11 illustrates anothergeometry of the mesa cladding layer. Two control electrodes 1110 and1120 are respectively formed on the sides of the mesa.

In yet another variation, the mesa strip cladding layer in theedge-emitting devices or the mesa cladding layer in the VCSELs may beeliminated. The lasers based on such structure are gain-guided. FIG. 12shows an example in which the driving current distribution provided bythe ohmic contact 120 on the cladding layer defines the optical gainregion and thus the optical modes. The control electrodes 1210 and 1220create depletion regions to control the carrier distribution.

Furthermore, the VCSELs may also be configured so that a laser beamexits the optical cavity from the substrate side of the device.

These and other variations are intended to be encompassed by theinvention as defined in the following claims.

What is claimed is:
 1. A method for constructing and operating asemiconductor device, comprising: providing an active medium layer of asemiconductor material that is responsive to electrical excitation toproduce photons, and first and second semiconductor cladding layersrespectively on first and second sides of said active medium layer toprovide an optical confinement; supplying a driving current across saidactive medium layer to effectuate said electrical excitation; forming anoptical cavity to enclose at least a portion of said active medium layerand to have a cavity optic axis substantially perpendicular to saidactive medium layer; and constructing a plurality of control electrodesseparate from one another relative to said active medium layer tocontrol said driving current to have a cross section profile along anelongated direction to define a polarization of said photons.
 2. Amethod as in claim 1, further comprising: doping said first claddinglayer to exhibit a first conductivity type; forming a pluralitysemiconductor regions of a second type of conductivity opposite to saidfirst conductivity type to form a plurality of p-n junctions, eachsemiconductor region being located underneath a respective controlelectrode; and adjusting control potentials to said control electrodesto reverse bias said p-n junctions to form depletion regions near saidp-n junctions.
 3. A method as in claim 1, wherein at least a portion ofat least one control electrode is directly in contact with a portion ofsaid second cladding layer to form a reverse biased Schottky contact. 4.A method as in claim 3, wherein said second cladding layer is n-doped,and further comprising: forming first and second ohmic contacts ofdifferent potentials respectively on said first and second sides of saidactive medium layer to supply said driving current; and setting controlpotentials on said control electrodes lower than a potential applied tosaid second ohmic contact.
 5. A method as in claim 3, wherein saidsecond cladding layer is p-doped, and further comprising: forming firstand second ohmic contacts of different potentials respectively on saidfirst and second sides of said active medium layer to supply saiddriving current; and setting control potentials on said controlelectrodes higher than a potential applied to said second ohmic contact.6. A method as in claim 1, further comprising: forming an oxide layer ona portion of said second cladding layer to allow at least a portion ofat least one control electrode in direct contact with said oxide layer.7. A method as in claim 6, further comprising: doping said secondcladding layer to have the n-type conductivity; forming first and secondohmic contacts of different potentials respectively on said first andsecond sides of said active medium layer to supply said driving current;and setting control potentials on said control electrodes lower than apotential applied to said second ohmic contact.
 8. A method as in claim6, further comprising: doping said second cladding layer to have thep-type conductivity; forming first and second ohmic contacts ofdifferent potentials respectively on said first and second sides of saidactive medium layer to supply said driving current; and setting controlpotentials on said control electrodes higher than a potential applied tosaid second ohmic contact.
 9. A method as in claim 1, further comprisingmodulating a potential applied to at least one of said controlelectrodes to produce a modulation in a number of said photons producedby said active medium layer.
 10. A method as in claim 1, wherein atleast one control electrode is directly in contact with a portion ofsaid active semiconductor medium layer to form a reverse biased Schottkycontact.
 11. A method as in claim 1, further comprising: forming anoxide layer on a portion of said active medium layer to be in contactwith at least a portion of at least one control electrode to form ametal-oxide capacitor.
 12. A method as in claim 1, wherein said activemedium layer includes a multiple quantum well structure.
 13. A method asin claim 1, further comprising adjusting at least two different controlpotentials applied to said control electrodes to reduce a spatialdistribution of said driving current in order to reduce a thresholdcurrent value for starting a laser oscillation in said optical cavity.14. A method for constructing and operating a semiconductor device,comprising: providing an active semiconductor medium layer of a typethat is responsive to electrical excitation to produce photons; forminga first and second optical reflectors respectively on said first andsecond sides of said active medium layer to form an optical cavity whichencloses at least a portion of said active medium layer and has a cavityoptic axis substantially perpendicular to said active medium layer;supplying a driving current across said active medium layer toeffectuate said electrical excitation and to produce a sufficient gainfor a laser oscillation; and applying at least two different controlpotentials to produce a plurality of depletion regions in said activemedium layer to cause said driving current to have a cross sectionalprofile along an elongated direction so that said laser oscillation ispolarized in said elongated direction.
 15. A method as in claim 14,further comprising adjusting said two control potentials to reduce aspatial distribution of said driving current in order to reduce a laserthreshold current for said laser oscillation.
 16. A method as in claim14, wherein at least one of said first and second optical reflectorsincludes alternating quarter-wavelength dielectric layers.